It is well known to employ photodiodes in a variety of light measurement apparatus. U.S. Pat. No. 4,521,106 discloses a range finder device useful, for example, in automatic focusing photographic cameras and including a linear array of photodiodes.
The general arrangement of elements and mode of operation of such a range finder device will be described with reference to FIG. 3. Means are provided for projecting a beam of light, for example an infrared light-emitting diode (LED) 10 and a lens 12, along a path 14 to illuminate a spot on an object 0.sub.1 in a scene. The scene is imaged by a second lens 16 onto a linear array of photodiodes 18. The signals produced by the photodiodes are analyzed by signal processing electronics 20 to determine the position of the illuminated spot in the scene and produce a signal representing the distance to the object.
As shown by example in FIG. 3, the apparent position of the illuminated spot in the scene is a function of the distance along light path 14 to the object. For an object 0.sub.1 located at a distance D.sub.1 from the range finder device, the image of the illuminated spot will fall on the photodiode array at location S.sub.1. For an object 0.sub.2 at a further distance D.sub.2, the image of the spot will fall on the photodiode array at location S.sub.2. By examining the output of the photodiode array, the control electronics 20 determines (for example by comparing the outputs of the photodiodes to determine that output which is a maximum) the location of the illuminated spot in the scene and thereby the distance to the object. To remove the effects of the scene brightness modulation, the range finder device is operated once with the LED on, and again with the LED off. The respective responses of the individual photodiodes are then subtracted from each other to obtain a signal representing only the illuminated spot. FIG. 4 shows a linear image sensor 18 useful with the range finder device. The image sensor 18 includes a linear array of photodiodes 22 and a charge coupled device (CCD) shift register 24. The CCD shift register has 2N cells labeled 1a, 1b, 2a, 2b, . . . Na,Nb. A transfer gate 26 between the array of photodiodes 22 and CCD shift register 24 is actuable for transferring photocharge (in the direction of arrow A) rom the photodiodes into alternate cells of the CCD shift register 24. A second transfer gate 32 between the photodiode array and a reset drain 28 is actuable for transferring charge on the photodiodes (in the direction of arrow B) into a reset diode 28. The CCD shift register is a buried channel CCD and the transfer channels created by the transfer gates 26 and 32 are surface channels.
The signals in the CCD shift register 24 are shifted serially in the direction of arrow C to an output diode 34 by applying four-phase clock signals .PHI..sub.1-4 to the transfer electrodes (not shown) of the CCD shift register The photosignals are detected at the output by a preamplifier 36.
In operation, the range finder device is operated once with the LED on, and the detected photosignals are transferred into alternate cells of CCD shift register 24. The photocharges are shifted one cell to the right (as seen in FIG. 4). Then the device is operated with the LED off, and the detected photosignals are likewise transferred into CCD 24. Photosignals are then shifted to the right as seen in FIG. 4 by applying four-phase clock signals .PHI..sub.1-4 to the transfer electrodes of the CCD shift registers. The photosignals are detected at the output diode 34 and amplified by preamplifier 36 before successive pairs are subtracted from each other by signal processing electronics 20.
Patent Application No. 797,093 filed on even date herewith by the present inventor discloses an improved method for introducing charge into the CCD shift register to provide a FAT ZERO. According to this method, the photodiodes 22 are employed as metering wells in a "time dependent" fill and spill process. The photodiodes are charged by pulsing the reset drain 28, and allowing charge to boil back over the potential barrier created by transfer gate T.sub.2 for a time t.sub.1. Then transfer gate T.sub.2 is raised and transfer gate T.sub.2 is lowered to the same level that T.sub.2 was previously lowered to, and charge is allowed to boil out of the photodiode over the potential barrier provided by transfer gate T.sub.1 for a time t.sub.2. The resulting charge Q transferred to the CCD is approximately: ##EQU1## where C is the capacitance of the diode, k is the Boltzmann constant (1.8.times.10.sup.-23 joules/degree Kelvin), T is the temperature in degrees Kelvin and q is the charge on an electron.
In developing this range finder device for use in a photographic camera, the inventor has found that an LED 10 producing a relatively long, narrow beam cross section (e.g. 100 .mu.m.times.1000 .mu.m) is particularly well suited for the light source. To optimize the signal-to-noise ratio of the range finder device, it is desirable that the photodiodes 22 have a size and shape similar to the size and shape of the beam cross section (e.g. 100 .mu.m.times.1000 .mu.m). Unfortunately, photodiodes of this size and shape have a much higher capacitance than the smaller more nearly square (e.g. 100.times.50 .mu.m) diodes used in the prior art device. As seen from equation (1), the amount of charge Q transferred to the CCD shift register during the generation of a FAT ZERO by the "time dependent" spill and fill process is directly proportional to the capacitance C of the photodiode. Because of the large capacitance of the larger photodiodes, the times employed in the time dependent spill and fill process become extremely short, and difficult to accurately control.
Accordingly, it is the object of the present invention to provide a photodiode structure having relatively large light sensitive area and a relatively low capacitance. A further object of the invention is to provide a linear image sensing array and an improved range finder device of the type described above, having such photodiodes.